Polycrystalline silicon thin film, fabrication method thereof, and thin film transistor without directional dependency on active channels fabricated using the same

ABSTRACT

A polycrystalline silicon thin film to be used in display devices, the thin film comprising adjacent primary grain boundaries that are not parallel to each other and do not contact each other, wherein an area surrounded by the primary grain boundaries is larger than 1 μm 2 , a fabrication method of the polycrystalline silicon thin film, and a thin film transistor fabricated using the method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 10/694,030 filedon Oct. 28, 2003, and claims the benefit under 35 U.S.C. § 119 of KoreanPatent Application No. 2003-13829 filed on Mar. 5, 2003. The contents ofapplication Ser. No. 10/694,030 and Korean Patent Application 2003-13829are hereby incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polycrystalline silicon thin film fordisplay devices, a fabrication method thereof, and a thin filmtransistor fabricated using the same. The present invention relates moreparticularly to a polycrystalline silicon thin film fabricated bycontrolling the shape of the silicon grains, a fabrication method of thethin film, and a thin film transistor fabricated using thepolycrystalline silicon thin film.

2. Description of the Related Art

Ordinarily, the sequential lateral solidification (SLS) method is usedfor crystallizing the silicon grains by overlappingly irradiating alaser beam onto an amorphous silicon layer two or more times so thatsilicon grains are laterally grown. Polycrystalline silicon grainsmanufactured using the SLS method are characterized in that they areformed in a cylindrical shape lengthy from end to end, and grainboundaries are generated between adjacent grains due to the limited sizeof the grains.

It is reported that polycrystalline or single crystalline grains arecapable of forming large silicon grains on a substrate using the SLScrystallization technology, and a thin film transistor (TFT) fabricatedusing the large silicon grains is capable of obtaining characteristicssimilar to characteristics of a TFT fabricated using single crystallinesilicon.

FIG. 1A, FIG. 1B, FIG. 1C, and FIG. 1D represent an ordinary SLScrystallization method.

In the SLS crystallization method, amorphous silicon is melted in thelaser beam transmission region if a laser beam is irradiated onto anamorphous silicon thin film layer through a mask having a laser beamtransmission region and a laser beam non-transmission region as shown inFIG. 1A.

Crystallization occurs on an amorphous silicon/molten silicon interfacewhen cooling begins after irradiation of the laser beam is completed,wherein a temperature gradient is formed in such a way that thetemperature of the silicon is gradually reduced from the amorphoussilicon/molten silicon interface toward the control part of the moltensilicon layer, the molten silicon solidifying, and crystallizing, as theheat dissipates.

Therefore, referring to FIG. 1B, a polycrystalline silicon thin filmlayer, having grains that are formed in a long cylindrical shape, isformed. Polycrystalline silicon grains are laterally grown until themolten silicon layer is completely solidified as heat flux flows fromthe mask interface to the central part of molten silicon layer.

As illustrated in FIG. 1C and FIG. 1D, by moving the laser beamtransmission region of the mask so that it exposes more of the amorphoussilicon and a portion of the crystalline silicon, and irradiating thisexposed area with the laser beam, the length of the grains is increased,with silicon atoms being adhered to already formed polycrystallinesilicon grains that are not melted, due to being covered by the mask, asthe partially melted amorphous silicon thin film and crystallizedsilicon layer are being cooled thereafter.

Therefore, TFT characteristics close to single crystalline silicon canbe obtained, since a barrier effect of a grain boundary to a chargecarrier direction is minimized in the case that an active channeldirection is parallel to the direction of the grains grown by the SLSmethod when fabricating the TFT. TFT characteristics are greatlydeteriorated in the case that the active channel direction isperpendicular to a grain growing direction, because a plurality of grainboundaries act as a trap of the charge carriers.

The mounting possibility of a circuit is restricted, because the TFTcharacteristics are greatly changed depending on the active channeldirection in the case of the TFT being fabricated by an existing SLSmethod.

On the other hand, it is disclosed in PCT International Publication No.WO97/45827 and U.S. Pat. No. 6,322,625 that amorphous silicon on thewhole substrate is converted into polycrystalline silicon, or only aselected region on the substrate is crystallized by SLS technology afterdepositing the amorphous silicon on a substrate.

Furthermore, when fabricating TFTs for a liquid crystal display (LCD)device, comprising a driver and pixel array, by forming large silicongrains using the SLS crystallization technology, characteristics of TFTssimilar to characteristics of TFTs fabricated using single crystallinesilicon can be obtained because the barrier effect of grain boundariesfor a charge carrier direction is minimized in the case that an activechannel direction is parallel to the direction of the grains grown bythe SLS crystallization method, as described in U.S. Pat. No. 6,177,391.But a plurality of grain boundaries which act as traps of the chargecarrier exist in this method, and also TFT characteristics are greatlydeteriorated in the case that the active channel direction isperpendicular to the growing direction of the grains in patents likethis.

Actually, there are cases in which TFTs inside a driving circuit aregenerally perpendicular to the TFT in a pixel cell region whenfabricating an active matrix display, wherein the uniformity of thedevice can be improved by fabricating the active matrix display in sucha way that the direction of the active channel regions is inclined tothe crystal growing direction at an angle of 30° to 60° in order toimprove the uniformity of characteristics between TFTs, while thecharacteristics of each TFT are not greatly deteriorated.

However, it is likely that fatal grain boundaries are included in theactive channel regions, since this method also uses grains having alimited size formed by the SLS crystallization technology. Therefore,there is a problem in that unpredictable non-uniformity causes adifference of characteristics between TFTs in this method.

SUMMARY OF THE INVENTION

Therefore, in order to solve the foregoing and/or other problems of therelated art, it is an aspect of the present invention to provide amethod for controlling the shape of polycrystalline silicon fabricatedin the case of fabricating a polycrystalline silicon thin film by theSLS crystallization method, and a polycrystalline silicon thin filmfabricated using the method.

Furthermore, it is another aspect of the present invention to provide aTFT having superior characteristics without the TFT characteristicsdepending on the active channel direction by using the above fabricatedpolycrystalline silicon thin film.

Additional aspects and advantages of the invention will be set forth inpart in the description which follows and, in part, will be obvious fromthe description, or may be learned by practice of the invention.

In order to achieve the foregoing and/or other aspects, the presentinvention provides a polycrystalline silicon thin film to be used indisplay devices, the thin film comprising adjacent primary grainboundaries that are not parallel to each other, wherein an areasurrounded by the primary grain boundaries is larger than 1 μm².

Furthermore, the present invention provide a thin film transistorfabricated using the polycrystalline silicon thin film.

Furthermore, the present invention provides a method of fabricating apolycrystalline silicon thin film to be used in display devices, themethod comprising crystallizing amorphous silicon by a laser using amask comprising a laser transmission region in which line shaped lasertransmission patterns and laser non-transmission patterns are mixed.

Furthermore, the present invention provides a fabrication method of apolycrystalline silicon thin film to be used in display devices, themethod comprising crystallizing amorphous silicon by a laser using amask in which laser transmission patterns are mixed with lasernon-transmission patterns, wherein the laser non-transmission patternsare circular or dot shaped opaque mask patterns.

Furthermore, the present invention provides a polycrystalline siliconthin film to be used in an electroluminescent display device fabricatedby the foregoing methods.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred embodiments, taken in conjunction with the accompanyingdrawings of which:

FIGS. 1A, 1B, 1C, and 1D are drawings schematically illustrating aconventional SLS crystallization method;

FIG. 2 is a plan figure schematically illustrating the structure of amask used in a polycrystalline silicon thin film fabrication methodaccording to an embodiment of the present invention;

FIG. 3 is a plan figure of the polycrystalline silicon thin filmfabricated using the mask of FIG. 2;

FIG. 4 is a plan figure schematically illustrating the structure of amask used in a polycrystalline silicon thin film fabrication methodaccording to another embodiment of the present invention;

FIG. 5 shows a mask pattern according to still another embodiment of thepresent invention, wherein a laser transmission region is formed in linepattern groups that are formed in a long rectangular shape in adirection, the line pattern groups are alternately arranged with beingparallel to each other by being spaced apart from each other at acertain distance, and a laser non-transmission dot shaped mask patternis arranged in a rectangular shape;

FIG. 6 shows grains of the polycrystalline silicon thin film fabricatedusing the mask of FIG. 5;

FIG. 7 illustrates a mask pattern according to yet another embodiment ofthe present invention, wherein the line pattern groups are formed in along rectangular shape in a laser transmission direction so that theline pattern groups are alternately arranged with being parallel to eachother by being spaced apart from each other at a certain distance, and alaser non-transmission dot shaped pattern is arranged in a triangleshape;

FIG. 8 shows grains of the polycrystalline silicon thin film fabricatedusing the mask of FIG. 7;

FIG. 9 illustrates a mask pattern according to yet another embodiment ofthe present invention, wherein the line pattern groups are formed in along rectangular shape in a laser transmission direction so that theline pattern groups are arranged perpendicularly to each other, and alaser non-transmission dot shaped pattern is arranged in an irregularrectangular shape;

FIG. 10 shows grains of the polycrystalline silicon thin film fabricatedusing the mask of FIG. 9;

FIG. 11 is a graph illustrating dependency on channel direction for anelectric field mobility of TFTs fabricated using the polycrystallinesilicon thin film fabricated according to embodiments of the presentinvention; and

FIG. 12 is a graph showing dependency on channel direction for thresholdvoltage of TFTs fabricated using the polycrystalline silicon thin filmfabricated according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tothe like elements throughout. The embodiments are described below inorder to explain the present invention by referring to the figures.

FIG. 2 is a plan figure schematically illustrating a structure of a maskused in a polycrystalline silicon thin film fabrication method accordingto an embodiment of the present invention, and FIG. 3 is a plan figureof the polycrystalline silicon thin film fabricated using the mask.

As shown in FIG. 2, polycrystalline silicon grains are formed in a shapewhich is long in a direction since heat flux is formed from the edge tothe central part of laser beam transmission patterns in the case that alaser beam is irradiated in such a way that the laser beam passesthrough a mask having line shaped patterns which are formed in a longrectangular shape in a direction.

In the case of using a mask pattern illustrated in FIG. 2, silicongrains that are grown oppositely to each other from the interface of alaser transmission part meet each other at a central part of patterns asshown in FIG. 3 so that grain boundaries having a high protrusion partare formed, resulting in the formation of cylindrical grains in whichprimary grain boundaries are arranged in a stripe shape.

FIG. 4 is a plan figure schematically illustrating a structure of a maskused in a polycrystalline silicon thin film fabrication method accordingto another embodiment of the present invention.

In FIG. 4, polycrystalline silicon grains are grown in every directionsince heat flux is generated from the central part of dot patterns,through which a laser beam is not capable of transmitting, to anexternal angle direction in the case of using a mask in which a laserbeam non-transmission part has circular or dot shaped patterns.

In the case that the dot patterns are arranged in a rectangular shape,polycrystalline silicon grains grown from the edge of the respective dotpatterns collide with each other so that primary grain boundaries formedaccordingly have a rectangular shape if a gap between the patterns isshorter than lateral growing distance of the grains. Therefore, apolycrystalline silicon thin film having various shaped microstructurescan be fabricated by controlling the shape and arrangement of the maskpatterns.

It may be preferable that adjacent primary grain boundaries of thepolycrystalline silicon grains are not parallel to each other, and thearea surrounded by the primary grain boundaries is larger than 1 μm².

FIG. 5 shows a mask pattern according to still another embodiment of thepresent invention, wherein a laser transmission region is formed in linepattern groups that are formed in a long rectangular shape in adirection, the line pattern groups are alternately arranged with beingparallel to each other by being spaced apart from each other at acertain distance, and a laser non-transmission dot shaped mask patternis arranged in a rectangular shape.

FIG. 6 shows the grains of the polycrystalline silicon thin filmfabricated using the mask of FIG. 5, wherein the primary grainboundaries of a polycrystalline silicon thin film are arranged in arectangular shape in the case of crystallizing amorphous silicon usingthe mask pattern of FIG. 5. It may also be preferable in this embodimentof the present invention that adjacent primary grain boundaries of thepolycrystalline silicon grains are not parallel to each other, and thearea surrounded by the primary grain boundaries is larger than 1 μm².

FIG. 7 illustrates a mask pattern according to yet another embodiment ofthe present invention, wherein the line pattern groups are formed in along rectangular shape in a laser transmission direction so that theline pattern groups are alternately arranged and are parallel to eachother by being spaced apart from each other at a certain distance, and alaser non-transmission dot shaped pattern is arranged in a triangleshape.

FIG. 8 shows grains of the polycrystalline silicon thin film fabricatedusing the mask of FIG. 7, wherein the primary grain boundaries of apolycrystalline silicon thin film are arranged in a hexagonal shape inthe case of crystallizing amorphous silicon using the mask pattern ofFIG. 7. It may also be preferable in this embodiment of the presentinvention that adjacent primary grain boundaries of the polycrystallinesilicon grains are not parallel to each other, and the area surroundedby the primary grain boundaries is larger than 1 μm².

FIG. 9 illustrates a mask pattern according to yet another embodiment ofthe present invention, wherein the line pattern groups are formed in along rectangular shape in a laser transmission direction so that theline pattern groups are arranged perpendicularly to each other, and alaser non-transmission dot shaped pattern is arranged in an irregularrectangular shape.

FIG. 10 shows grains of the polycrystalline silicon thin film fabricatedusing the mask of FIG. 9, wherein the primary grain boundaries of apolycrystalline silicon thin film are arranged in an irregular closedpolygonal shape in the case of crystallizing amorphous silicon using themask pattern of FIG. 9. It may also be preferable in this embodiment ofthe present invention that adjacent primary grain boundaries of thepolycrystalline silicon grains are not parallel to each other, and areasurrounded by the primary grain boundaries is larger than 1 μm².

The primary grain boundaries of the polycrystalline silicon grainsfabricated in these embodiments of the present invention are symmetricalto each other centering around a certain axis passing through theprimary grain boundaries, and the primary grain boundaries preferablyform a hyperbola centering around a radial shape or the certain axis.

It may be preferable that a thin film transistor fabricated using apolycrystalline silicon thin film of the present invention is used in anorganic electroluminescent display device.

FIG. 11 is a graph illustrating dependency on channel direction forelectric field mobility of a TFT fabricated using the polycrystallinesilicon thin film fabricated using a mask pattern according toembodiments of the present invention illustrated in FIG. 3 and FIG. 9.

FIG. 12 is a graph illustrating dependency on channel direction forthreshold voltage of a TFT fabricated using the polycrystalline siliconthin film fabricated using a mask pattern according to embodiments ofthe present invention illustrated in FIG. 3 and FIG. 9.

Higher electric field mobility and lower threshold voltagecharacteristics were obtained due to the small number of grainboundaries if primary grain boundaries of the polycrystalline siliconthin film of FIG. 3 were perpendicular to an active channel direction ofthe TFT. If the active channel direction of the TFT was parallel to theprimary grain boundaries, the TFT characteristics were greatlydeteriorated, due to the large number of grain boundaries, so that theelectric field mobility was reduced by 60 % or more, and the thresholdvoltage was increased by 60% or more.

On the other hand, dependency of the TFT characteristics on the channeldirection was greatly deteriorated so that the difference of the TFTcharacteristics could be controlled within the range of 25% depending onthe channel direction in the case of a polycrystalline silicon thin filmtransistor fabricated using the mask of FIG. 9.

Therefore, the present invention is capable of fabricating apolycrystalline silicon thin film having various shaped grain structuresby crystallizing amorphous silicon using a mask in which line shapedpatterns are mixed with dot shaped patterns, and fabricating a thin filmtransistor having superior characteristics without dependency on achannel direction by controlling the microstructure of thepolycrystalline silicon thin film through the design of the mask,thereby improving the degree of integration of a circuit part on a panelthrough the fabricated thin film transistor.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A polycrystalline silicon thin film to be used in display devices,the thin film comprising adjacent primary grain boundaries that are notparallel to each other and do not contact each other, wherein an areasurrounded by the primary grain boundaries is larger than 1 μm².
 2. Thepolycrystalline silicon thin film according to claim 1, wherein theprimary grain boundaries are formed in a closed curve shape or a closedpolygonal shape.
 3. The polycrystalline silicon thin film according toclaim 1, wherein the primary grain boundaries are formed in arectangular or a hexagonal shape.
 4. The polycrystalline silicon thinfilm according to claim 1, wherein the primary grain boundaries aresymmetrical to each other centering around a certain axis passingthrough the primary grain boundaries.
 5. A thin film transistorfabricated using the polycrystalline silicon thin film according toclaim
 1. 6. The thin film transistor according to claim 5, wherein thethin film transistor is used in an organic electroluminescent displaydevice.
 7. A polycrystalline silicon thin film to be used in displaydevices, the thin film comprising primary grain boundaries that are notparallel to each other and do not contact each other.
 8. Thepolycrystalline silicon thin film according to claim 7, wherein theprimary grain boundaries are formed in a closed curve shape or a closedpolygonal shape.
 9. The polycrystalline silicon thin film according toclaim 7, wherein the primary grain boundaries are formed in arectangular or a hexagonal shape.
 10. The polycrystalline silicon thinfilm according to claim 7, wherein the primary grain boundaries aresymmetrical to each other centering around a certain axis passingthrough the primary grain boundaries.
 11. A thin film transistorfabricated using the polycrystalline silicon thin film according toclaim
 7. 12. The thin film transistor according to claim 11, wherein thethin film transistor is used in an organic electroluminescent displaydevice.